The field of horizontal direction drilling (HDD) has witnessed many advancements over the years. HDD has been widely used to create non-vertical wells in oilfields, to install and repair utility lines, and in various types of mining. HDD permits access to areas that would be inaccessible by conventional vertical wells. HDD also allows multiple wellheads to be grouped together and also permits relief wells to be more easily made. Despite the many advantages to HDD, there are many significant disadvantages.
One significant drawback is the frequency with which drillstrings become unstable. Drillstring instability can manifest itself in a variety of ways, including, but not limited to, loss of directional control, loss of angular control, loss of forward movement, and buckling. Drillstring instability can severely limit the range of HDD operations. If too high a thrust is applied the drilling head may stop and the drillstring may buckle and become locked into the ground. Drillstring instability may also occur when exceeding the maximum curvature of the drillstring. Regardless of the cause, the result is the same. The operator must back out the drillstring, or worse yet, the drillstring must be excavated. As a consequence, operators of HDD rigs spend years developing the “feel” necessary to determine when a drillstring is potentially becoming unstable.
Still yet other problems result from “cross-bore,” or the drilling through an existing gas pipe or utility line. These cross-bore incidents reflect the fact that HDD operations are conducted in the blind. These cross-bore incidents are often the result of poor mapping of utility and/or gas lines. Proximity sensors are theoretically possible but are not practical due to cost considerations. Additionally, maps of underground utilities are notoriously unreliable and outdated. The consequences of cross-bore events can be costly and, in some instances, fatal.
As a result of the foregoing, efforts have been made over the years to accurately determine the position of the drilling head during HDD operations. One such effort is described in U.S. Pat. No. 5,193,628 to Hill, III et al. Hill discloses a position determining system. The system and method are termed “POLO,” or POsition LOcation technology. The system successively and periodically determines the radius of curvature and azimuth of a portion of a drill pipe from axial strain measurements made on the outer surface of the drill pipe. Using these determinations, the system constructs on a segment-by-segment basis, circular arc data representing the path of the borehole. The location of the drilling head can also be obtained. Yet another effort is described in U.S. Pat. No. 7,584,808 to Dolgin et al. Dolgin discloses a centralizer based survey and navigation system (CSN). The system is designed to provide borehole or passageway position information via displacement sensors, centralizers, an odometer, a borehole initialization system, and a navigation algorithm implementing processor.
Although these position determining systems achieve their own particular objectives there still exists a need to determine potential drillstring instability during HDD operations. There also exists a need to provide a system whereby soil characteristics can be determine during HDD operations. A further need exists to tell drillstring operators when an old bore, or disturbed soil around an old bore, is encountered. The system and method disclosed herein are aimed at fulfilling these and other needs.